Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of various advantages such as its non-impact, low noise characteristics and system simplicity. For these reasons, ink jet printers have achieved commercial success for home and office use and other areas.
Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the printhead. A slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
In continuous stream or continuous inkjet printing, a pressurized ink source is used for producing a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no printing is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. While such continuous inkjet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics, the electrostatic deflection mechanism they employ is expensive to manufacture and relatively fragile during operation.
Recently, a novel continuous ink jet printer system has been developed which renders the above-described electrostatic charging devices unnecessary and provides improved control of droplet formation. The system is disclosed in the commonly assigned U.S. Pat. No. 6,079,821 in which periodic application of weak heat pulses to the ink stream by a heater causes the ink stream to break up into a plurality of droplets synchronous with the applied heat pulses and at a position spaced from the nozzle. The droplets are deflected by increased heat pulses from a heater in a nozzle bore. This is referred to as asymmetrical application of heat pulses. The heat pulses deflect ink drops between a “print” direction (onto a recording medium), and a “non-print” direction (back into a “catcher”). Although solvent-based inks such as alcohol-based inks have quite good deflection patterns and achieve high image quality in asymmetrically heated continuous ink jet printers, water-based inks do not deflect as much, and consequently, their operation is not as robust.
Still other methods of continuous ink jet printing employ air flow in the vicinity of ink streams for various purposes. For example, U.S. Pat. No. 3,596,275 discloses the use of both collinear and perpendicular air flow to the droplet flow path to remove the effect of the wake turbulence on the path of succeeding droplets. This work was expanded upon in U.S. Pat. No. 3,972,051, U.S. Pat. No. 4,097,872, and U.S. Pat. No. 4,297,712 in regards to the design of aspirators for use in droplet wake minimization. U.S. Pat. No. 4,106,032 and U.S. Pat. No. 4,728,969 employ a coaxial air flow to assist jetting from a drop-on-demand type head.
One problem associated with inkjet printers in general and such printers employing gas or air flows in particular is the drying of the ink. Ink drying in the vicinity of the printhead nozzles can lead to spurious droplet trajectories and nozzle clogging which in turn complicate the proper deflection of the print droplets. Additionally, the evaporation of the ink solvent from the droplets as they fly through the air can increase the viscosity of the ink captured by the gutter, thereby causing difficulties during the ink recycling operation when the recycled ink is passed through a filter.
Clearly, there is a need for a continuous ink jet method and printing apparatus with a simpler, less expensive and more robust ink deflection or control mechanism that does not employ air flows in the vicinity of the nozzles. In particular, it would be desirable to provide such a continuous ink jet method and printing apparatus that does not rely upon the electrostatic devices or heater devices for deflection purposes, and that does not employ air flow to avoid the expenses, limitations and disadvantages associated with each of these different technologies.